It is known to manufacture hollow metallic articles by diffusion bonding and superplastic forming metal workpieces. These metal workpieces include elementary metal, metal alloys, intermetallic materials and metal matrix composites.
The diffusion bonding and superplastic forming process may be used to produce contoured articles for example fan blades, or fan duct outlet guide vanes, for gas turbine engines by superplastically, or hot forming, an integral structure formed by the diffusion bonding process.
A procedure for manufacturing an article by diffusion bonding and superplastic forming is disclosed in our European patent EP0568201B. In EP0568201B the integral structure formed by the diffusion bonding process is twisted before the integral structure is superplastically formed. Additionally the integral structure is hot creep formed in the superplastic forming dies.
Additionally our UK patent GB2306353B discloses manufacturing a fan blade by diffusion bonding and superplastic forming. In GB2306353B the integral structure is formed from two metallic workpieces which subsequently define the outer profile of the fan blade. The two metallic workpieces are produced by cutting an inclined slot through a parallelepiped metal block to produce two longitudinally tapering metallic workpieces. The thicker ends of the metallic workpieces are aligned to form the root of the fan blade and the remainder of the metallic workpieces are machined to the appropriate thickness to give the required mass distribution.
This manufacturing process requires that the thickness of the original parallelepiped metallic block is about half, just less than half, of the thickness of the root of the finished fan blade in order to allow machining to produce the root. A problem with this process is that it is wasteful of metal, machining time and is expensive. Additionally the microstructure of the parallelepiped metallic block is not the optimum microstructure, due to the thickness of the original metallic block.
The problem is partially overcome, as also disclosed. in GB2306353B, by using thinner parallelepiped metallic blocks and adding extra small metallic blocks at the thicker ends of the two longitudinally tapering metallic workpieces to form the root of the fan blade. The metallic blocks are initially welded to the parallelepiped metallic blocks to seal them together before they are diffusion bonded together. The welding process introduces a “heat affected zone” in which material properties are compromised. This gives a minimum thickness of the parallelepiped metallic blocks from which the fan blade may be manufactured because the properties of the metal may not be degraded on the surfaces of the fan blade root.